1. Field of the Invention
The present invention relates broadly to fluid seals and, more particularly, to fluid seals which may be suitably used in hydraulic shock absorbers wherein a gas under pressure is charged.
2. Description of the Prior Art
As is well-known, automotive suspension systems are generally equipped with hydraulic shock absorbers adapted to dampen the shock as the wheel hits a bump and to attenuate oscillation of the suspension springs. During operation of the shock absorbers, the hydraulic fluid absorbs the energy applied to the suspension systems and converts it into heat. The hydraulic fluid also undergoes strong agitation as it flows across orifice of the valved piston. Under severe operating conditions, heat and agitation lead to formation of vapor bubbles which would impair the performance of the shock absorbers.
To suppress the formation of or reduce the effect of vapor bubbles, modern shock absorbers are generally charged with an inactive gas, such as nitrogen gas, which is under pressure in the range of 4-6 kg/cm.sup.2.
In a gas-filled shock absorber, a fluid seal for sealing the piston rod or shaft relative to the shock absorber housing is constantly subjected to the gas pressure. Accordingly, an essential requirement for the shaft seals for gas-filled shock absorber applications is to effectively prevent leakage of the hydraulic fluid as well as gas under pressure throughout the service life of the shock absorbers.
The shock absorbers are also subjected during operation to a substantial lateral thrust. In particular, a severe lateral thrust is encountered in the MacPherson strut type suspension systems. The lateral thrust is increased with increasing speed performance of the automobiles. Although the piston rod of the shock absorber is guided by a rigid guide supported by the housing, the lateral thrust causes the piston rod to offset to some degree relative to the housing. This brings about an uneven sealing pressure distribution over the inner circumference of the shaft seal. In the region where the sealing pressure is reduced, there is a likelihood of leakage of the hydraulic fluid and gas under pressure.
It is therefore an object of the present invention to provide a shaft seal having a high degree of sealing capability.
Another important requirement for the shaft seals for shock absorber applications is somewhat opposing. It is desirable that the shaft seals undergo reduced wear and drag (dynamic friction).
Accordingly, another object of the invention is to provide a fluid seal which is capable of being adequately lubricated by the hydraulic fluid without sacrificing the sealing capability.